This invention relates to a method and apparatus for separation of relatively heavy and light particles from particulate material and is particularly, although not exclusively, applicable for the separation of objectionable particles from tobacco material, for example from cut or uncut tobacco. Such objectionable particles may be "heavies", for example, coarse cut stem pieces and/or "lights", for example particles of dust. The invention can however also be applied to other particulate materials such as vegetables, paper, and mineral materials, or any other particulate material which requires separation. The invention will be described, however, with respect to tobacco materials.
In processing, different tobacco components are treated in different ways before being combined to form the final blend. For example, lamina undergoes a different conditioning treatment to stem and is cut more finely. If there is some cross-contamination of tobacco type such as stem on lamina or lamina on stem, problems will occur after cutting. After cutting, some of the stem in lamina will be so coarsely cut it will be deemed to be objectionable and some of the lamina will be so finely cut it will, in the latter stages of processing, be rendered to dust. For the maintenance of quality, both the overtly large and small particles must be removed from the tobacco prior to being manufactured into the cigarette rod.
One way of removing objectionable particles has been provided for in cigarette making machines, in that prior to forming the unwrapped cigarette rod, the tobacco in the machines is passed through a winnower and air lifted. In passing through the winnower , some heavy objectionable particles are removed. In air lifting, some of the dust passes through the machines to be removed by filtration before the air is exhausted to the atmosphere . Both of these processes are inefficient and remove only a portion of the objectionable material present. Their efficiency is also load dependent, that is, the more objectionable material present, the lower their efficiency. Their discrimination of the winnows is also poor, resulting in acceptable material being rejected with the objectionable.
Another method of removing objectionable material for example is to classify it out by air lifting. These are several styles of classification in existence . These work on the principle that the heavy particles can be separated from the light particles by passing them through a moving stream of air which carries the light particles off with it for separation later, while the heavy particles due to their mass/aerodynamic qualities are left behind.
As the light particles are usually the acceptable and less robust port ion of the tobacco and the air velocities used are in the order of 3,000 ft/min or higher , this form of separation usually results in some degradation of the good tobacco components . Again discrimination between heavy and light particles is poor due to the aerodynamic shadowing and the very short time in which separation occurs .
U.S. Pat. No. 4,646,759 shows apparatus for the separation of tobacco into two fractions, for example "heavies" and "lights". The tobacco is supplied to a separator unit including a vibrating conveyer and streams of air rising through the conveyer plate lift the lighter particles away. The particles most desirable for use as cigarette filler are pulled away and into an upper collector chamber and there deposited into a collector tray leaving the heavy particles to be discharged separately.
The present Applicants attempted to overcome some of the objectionable aspects of the arrangements referred to above and in their corresponding European Patent Application No. 89309703.0 (Publication No. 0 361 815, published Apr. 4, 1990) which shows a method of separating objectionable particles from host tobacco material and apparatus for carrying out the method are described. The apparatus comprises means for fluidising the material to form a carpet in an air stream, means for simultaneously agitating the material to release the dust and heavy particles and arranging the air flow velocity of the air stream to cause the dust to rise and the heavy particles to sink from the carpet. Means are provided for removing the dust and the heavy particles. Further research into this method and equipment have shown that the stratified air velocities within and over the deck which form the carpet of material can be more efficiently produced by control of the air entering the troughs and the present invention is therefore intended to provide a more efficient apparatus of the kind referred to in European Patent Application No. 89309703.0.